4 While continuing to beat the egg whites, slowly add sugar, then vinegar, and beat to stiff peaks: Increase the speed to medium-high, and slowly add the sugar, a couple tablespoons at a time, to the egg whites. Continue to whip the eggs and sugar for a few minutes.
Then add the vinegar to the bowl. Increase speed to high and whip the egg whites until they fluff up and become glossy, and stiff peaks form when the whisk is lifted, 4-5 minutes.
5 Fold in pecan pieces: Using a rubber spatula, gently fold in the pecan pieces.
6 Drop by teaspoons onto a cookie sheets that have been lined with parchment paper or a silicone mat.
7 Bake: Put the cookies sheet in the 300°F (150°C) oven, close the door and lower the heat to 250°F (120°C). Bake them for 25 minutes at 250°F (120°C), then turn the oven OFF. Leave them in the oven for 2 to 3 hours or overnight. When they are ready they'll be crisp on the outside, and light and airy on the inside.
If they are a little marshmallowy or chewy on the inside, just let them dry out for a few more hours.
Prep time: 20 minutes
Cook time: 25 minutes
Oven cooling time: 3 hours
Yield: Makes 12-24 cookies, depending on the size.